1. Field of the Invention
The present invention relates to a preheating method and apparatus for preheating raw tire prior to vulcanization molding process.
2. Description of the Related Art
Raw tire is conventionally shaped by molding machine, and kept on rack of storage space, which is designated in a building etc. of vulcanizing facilities, in a room-temperature environment. The raw tire is subsequently gotten out of the rack based on production planning, and conveyed to vulcanizing machine. Bladder, which is inserted into the tire hole, is stretched to be brought into intimate contact with the inner wall surface of the tire by supplying the bladder with high-temperature and high-pressure heating medium after the raw tire, which is carried in, for example, a bladder-type vulcanizing machine, being loaded into a mold by means of clamping action. Then, the inner wall surface of the tire is heated and pushed in the direction of the mold through the bladder to form mold tire grooves on the tread portion of the tire. Further, vulcanization molding process may be completed in a short time by using a heated mold and a bladder contacted with high-temperature heating medium to heat the raw tire from the outside and the inside respectively, which results in early temperature rise up to the vulcanizing start temperature.
However, to keep raw tire for a long stretch of time in a room-temperature environment in accordance with the prior art above, which results in that the temperature of the raw tire becomes close to the room temperature of 25 degrees Celsius etc., requires temperature rise of the raw tire from the room temperature up to the vulcanizing start temperature in a vulcanization molding process of the tire in a vulcanizing machine. Although raw tire is conventionally heated from the outside and the inside by using mold or bladder respectively to complete vulcanization molding process in a short time as mentioned above, the temperature on the inner side and, more particularly the center on the inner side of notably fleshier part of the tread portion or the bead portion of the raw tire rises slowly while the temperature of the outer surface side of the raw tire rises in a short time, because raw tire is mainly composed of rubbers with low thermal conductivity. Therefore, the vulcanization molding process is required to be continued until the temperature on the inner side of the raw tire reaches the vulcanizing start temperature to carry out vulcanization even if the outer surface side of the raw tire has already finished vulcanization. On this account, the method of the prior art suffer from problem in that vulcanization molding may not be completed adequately in a short time.
In addition, microwave may be irradiated to raw tire while being stored as a preheating method of raw tire prior to vulcanization molding process. However, this method may not be a fundamental solution for completing the vulcanization molding process in a short time because microwave of the method may preheat (heat) the outer surface side of the raw tire mainly, but not the inner side, the temperature of the side rising most slowly while vulcanization molding. Furthermore, the rate of absorption of microwave energy is sensitive to a slight dispersion of composition of rubber members, the tendency of which becomes more pronounced as the heating temperature rises, and makes it difficult to heat raw tire evenly. Then, vulcanizing machines of every kind such as bladder-type, bladderless-type etc. suffer from this kind of problem, particularly notable for the bladder-type vulcanizing machine because bladder itself is composed of rubbers with low thermal conductivity.